1. Field of the Invention
The present invention relates to a planetary gear drive installed in a multistage automatic transmission.
2. Description of the Prior Art
Usually, a planetary gear drive of an automatic transmission uses two planetary gearsets in series to obtain three or four forward speeds.
Nowadays, however, for achieving a much smoother speed change, more than four forward speeds have been required particularly in the field of high-class cars. For achieving this purpose three planetary gearsets are usually employed. However, in this case, the transmission inevitably becomes bulky.
In order to solve this drawback, an idea has been proposed by U.S. Pat. No. 2,873,625 in which six forward speeds are obtained by only two planetary gearsets.
In order to clarify the task of the invention, the planetary gear drive of the U.S. Patent indentified above will be outlined with reference to FIG. 4 of the accompanying drawings.
The gear drive comprises two coaxially arranged planetary gearsets P11 and P12 which are interposed between input and output shafts "I" and "O". The gearsets are of a single pinion type. The power transmission from the input shaft "I" to the output shaft "O" is achieved by selectively operating four clutches C11, C12, C13 and C14 and two brakes B11 and B12.
Table-1 shows the relation between the conditions of the clutches and the brakes, and the speed selected.
TABLE 1 ______________________________________ Speed selected C11 C12 C13 C14 B11 B12 ______________________________________ Forward 1'st speed .smallcircle. x x .smallcircle. x .smallcircle. Forward 2'nd speed .smallcircle. x .smallcircle. x .smallcircle. .smallcircle. Forward 3'rd speed .smallcircle. x x .smallcircle. .smallcircle. x Forward 4'th speed .smallcircle. x .smallcircle. x .smallcircle. x Forward 5'th speed x .smallcircle. .smallcircle. .smallcircle. x x Forward 6'th speed x .smallcircle. .smallcircle. x x x Reverse .smallcircle. .smallcircle. x x x .smallcircle. ______________________________________ Note: .smallcircle.: ON (engaged) x: OFF (disengaged)
However, due to its inherent construction, the above planetary gear drive has the following drawbacks.
That is, as is understood from Table-1, when the switch between 2'nd and 3'rd speeds, and between 4'th and 5'th speeds is accomplished four ON-OFF switch operations are necessary in the associated cluthes and brakes. However, these four ON-OFF switch operations tend to make a smooth torque change of the output shaft difficult. That is, non-negligible shift shock tends to occur awing such speed changes.
Even when one speed for which the four ON-OFF switch operations are needed is removed to make the gear drive have "five" forward speeds, the remaining one speed causes the undesired shift shock.